This relates to package design and more particularly to patterns of electrical interconnects that are used for high performance devices such as high speed differential signaling transceiver pairs and memory interfaces. As is known in the art, such interfaces are typically implemented using a ball grid array (BGA) or a pin grid array (PGA) A BGA is an area array of solder balls or solder bumps that is located on the surface of a package. A PGA is an area array of pins underneath a package surface. The BGA or PGA is used to connect the package to the next level of package. See, R. R. Tummala, Fundamentals of Microsystems Packaging, pp. 67, 68, 279-281, 680-682, 925 (McGraw-Hill, 2001), which is incorporated by reference herein in its entirety. For convenience, the term “contacts”or “interconnects” will be used hereafter to refer to solder balls, solder bumps and interconnect pins as well as similar connectors.
In designing high speed differential signaling interfaces, it is important to achieve nominal differential impedance and good isolation between the high speed differential signaling interconnects. The practice has been to use as many ground contacts as possible to surround each pair of high speed differential signaling contacts. This, however, results in the use of large numbers of ground contacts, leading to consequences such as the need for trade-offs between the number of ground contacts and the number of I/O contacts, trade-offs in the number of ground contacts versus performance, and/or increases in the size of the interconnect package.
FIGS. 1A and 1B depict two interconnect arrangements in which each differential signaling pair is surrounded by ground contacts. As shown in FIG. 1A, a rectilinear array 100 of contacts comprises ground contacts 102, pairs of differential signal receiving contacts 104 and pairs of differential signal transmitting contacts 106 extending along an edge 110 of an interconnect package. As will be apparent, each differential pair of contacts 104 or 106 is surrounded by ten ground contacts 102. The pattern shown in FIG. 1A is a repeating pattern that is made by stepping and repeating the pattern of twelve contacts defined by rectangular box 108. As will be apparent, the rectangular box contains two pairs of signaling contacts 104, 106 and eight ground contacts for a signaling to ground ratio of 2:8.
As shown in FIG. 1B, a rectilinear array 120 of contacts comprises ground contacts 122, pairs of differential signal receiving contacts 124 and pairs of differential signal transmitting contacts 126 extending along an edge 130 of a BGA package. As will be apparent, each differential pair of contacts 124 or 126 is surrounded by six ground contacts 122. The pattern shown in FIG. 1B is a repeating pattern that is made by stepping and repeating the pattern of eight contacts defined by rectangular box 128. As will be apparent, the rectangular box contains two pairs of signaling contacts 124, 126 and four ground contacts 122 for a signaling to ground ratio of 2:4. This reduction in the number of grounds contacts by 50% is achieved with very little impact on either the differential impedance or the coupling coefficient.
Efforts at further reduction in the number of ground contacts have not been as successful. FIG. 1C depicts one such effort in which a rectilinear array 140 of contacts comprises ground contacts 142, pairs of differential signal receiving contacts 144 and pairs of differential signal transmitting contacts 146 extending along an edge 150 of an interconnect package. The pattern shown in FIG. 1C is a repeating pattern that is made by stepping and repeating the pattern of twelve contacts defined by the parallelogram 148. As will be apparent, the parallelogram contains four pairs of signaling contacts 144, 146 and four ground contacts for a signaling to ground ratio of 2:2. In some implementations, the two ground contacts in the row immediately adjacent edge 150 are not present in order to improve access to signal transmitting contacts 146. While the pattern of FIG. 1C has achieved an additional reduction of 50% in the number of ground contacts, it comes at a cost. Circuit simulations indicate that the differential impedance of the pattern with two ground contacts increases about 10% and the coupling coefficient is about 15× higher than that of the pattern of FIG. 1A.